In the various natural gas deposits containing gases with higher portions of hydrogen sulfide the reservoir rock is more or less impregnated with elementary sulfur. In these cases, gas collected at the well head contains elementary sulfur in an amount which is dependent upon pressure, temperature and composition. The solubility of the elementary sulfur in the gas increases with the content of hydrogen sulfide, the content of carbon dioxide, the temperature and pressure. The hydrogen sulfide content of acidic natural gases varies greatly. Hydrogen sulfide containing gases which will entrain appreciable amounts of elementary sulfur generally have an H.sub.2 S content between 5 and 30% by volume. Normally, they also contain CO.sub.2 in the same amount.
When mining the gas, its pressure and temperature decreases during the passage through the riser. The decrease of temperature and pressure is accompanied by a corresponding reduction in the solubility of the sulfur. The elementary sulfur precipitates and adheres to the wall of the riser in certain zones. If there is a major precipitation of the sulfur the riser is blocked in such a short time that mechanical measures for removing the precipitated sulfur are unsatisfactory because they are either too costly or the interruption of operations is too lengthy. Therefore means for the continuous dissolution of the precipitating sulfur have long been sought.
Several suggestions have been made to solve this technical problem. The principal measures employed in the mining technology are: (a) dissolution of the entrained elementary sulfur in aqueous solutions in the form of polysulfides (German Patent Specification No. 1 286 586), (b) dissolution of the entrained sulfur in high boiling hydrocarbons and dialkyl sulfides which are immiscible with the precipitating condensation water and the entrained water of the deposit respectively (German Patent Specification No. 1 173 856, U.S. Pat. No. 3,846,311).
The solvents, such as an ethylamine solution, spindle oil etc., are added to the rising natural gas stream at the mouth of the riser. After the gas exits the outlet of the well, the resulting sulfur solutions are separated by means of usual liquid separating agents. These same separation means may be employed in association with the process of this invention.
For the dissolution of the entrained sulfur in the form of polysulfides, low molecular weight amines, such as methylamine, dimethylamine, ethylamine, etc., are particularly suitable. The polysulfides may be dissociated by the removal of the amine and the hydrogen sulfide at about 140.degree. C. with the formation of liquid sulfur. Amine and hydrogen sulfide are condensed together with the resultant steam and returned into the probe (German Patent Specification No. 1 286 586).
When higher amounts of deposit water containing calcium salts are entrained precipitates of calcium carbonate may result from the addition of amine solutions in the presence of CO.sub.2 in the gas. These precipitates impair the mining operations and block the equipment.
In such cases water-insoluble organic colvents may be used. As hydrocarbons having an intermediate molecular weight are appreciably soluble in natural gas at the relatively high pressures in the deposits and during mining, hydrocarbons of high molecular weights which dissolve in the gas to a smaller extent are necessary. Suitable high molecular weight hydrocarbon fractions are, for instance, obtained as spindle oils during the vacuum distillation of mineral oils. The regeneration of the spindle oil is effected by extracting the sulfur utilizing aqueous solutions of sodium sulfide and sodium hydrogen sulfide respectively to produce sodium polysulfide.
A disadvantage when dissolving the entrained sulfur in the spindle oil is the low solubility of the sulfur at room temperature and the resulting high dependence on the temperature for operation of the process. At 30.degree. C. only about 1% by weight of sulfur dissolves, while about 6% by weight dissolves at 90.degree. C.
Clearly, it is desirable to limit the amount of circulating solvent. Therefore there is the need for an improved solvent for sulfur at low temperature.